There is a desire to form an engine main body defining a cylinder and a crankcase chamber by using as small a number of component parts as possible, and support the crankshaft of the engine in a favorable manner in terms of structural integrity and ease of assembly. In a known overhead camshaft engine disclosed in JP2002-349340A, the engine main body includes a cylinder block that is integrally formed with a cylinder head, and a first bearing for supporting a first end of the crankshaft is fitted into a hole formed in the cylinder block. The lower end of the cylinder block is defined by a plane extending obliquely across the crankshaft, and a crankcase having a corresponding upper end is attached to the lower end of the cylinder block to define the crankcase chamber in cooperation with the cylinder block. The crankcase is formed with a hole, and a second bearing for supporting a second end of the crankshaft is fitted into the hole formed in the crankcase.
This engine main body essentially consists of two pieces. However, the crankcase is required to be made of relatively stiff member, and the assembling of the engine may not be as easy as wished. Also, because the crankshaft pulley for transmitting the power of the crankshaft to the cam mechanism via a timing belt is positioned on the part of the crankshaft located inward of the second bearing, the distance between the two bearings is increased by the width of this crankshaft pulley, and this is detrimental in maximizing the stiffness of the crankshaft. Also, the positioning of the timing belt complicates the cooling system of the engine.